1. Field of the Invention
The present invention relates to an ink droplet ejection device for a drop-on demand type printer, and more particularly to an ink droplet ejection device operable in accordance with deformation of a piezoelectric transducer.
2. Description of the Prior Art
Recently, there has been proposed a printer using a piezoelectric ink jet head. Such type of printer is specifically called a drop-on demand printer in which an ink droplet is ejected from an orifice when a volume of an ink channel is decreased owing to the inward deformation of a piezoelectric transducer while ink is supplemented into the ink channel through a valve when the volume of the ink channel is increased owing to the outward deformation of the piezoelectric transducer. A plurality of such ejection devices are juxtaposed to provide an ejection unit and ink droplets are ejected from selected ejection devices to print a desired character, graphic or image.
Ink droplet ejection devices of the type described above are disclosed in Japanese Laid-Open Patent Publications Nos. 63-247051 and 63-252750. Such a prior art device will be described with reference to FIGS. 1, 2 and 3.
Referring first to FIG. 1, an ejection device array includes a piezoelectric ceramic plate 1 and a cover plate 2. The piezoelectric ceramic plate 1 is formed with a plurality of protrusions 13a through 13d on one surface thereof and is polarized in a direction of its thickness indicated by an arrow d. The cover plate 2 is made of such a material as metal, glass, ceramics or resin. The piezoelectric plate 1 and the cover plate 2 are bonded together with an adhesive material 5 to form horizontally spacedly arranged ink channels 31a, 31b and 31c. The ink channels 31a, 31b and 31c are defined by the cover plate 2 and the protrusions 13a through 13d serving as side walls of the ink channels. Each ink channel is rectangular in cross-section and extends in a direction perpendicular to the sheet of drawing. Each of the side walls 13a through 13d is deformable in a direction perpendicular to both the polarization direction and the longitudinal direction of the ink channel, thereby changing an ink pressure in the ink channel. Metal electrodes, such as 11a through 11d, are formed on the surface of the side walls 13a through 13d, respectively. Driving electric field is selectively applied to the metal electrode to actuate the corresponding ejection device. The metal electrodes 11a through 11d are subjected to surface treatment to inhibit corrosion by the ink.
The ejection device array thus constructed will operate in such a manner that when the ejection device 31b is selected by print data, a driving electric field is applied between the metal electrodes 11a and 11b and between 11c and 11d. Since the electric field direction and the polarization direction are orthogonal to each other, both the side walls 13b and 13c are inwardly deformed with respect to the ink channel 31b as shown in FIG. 2 according to a piezoelectric thickness shearing effect. This inward deformations of the side walls 13b and 13c reduces the inner volume of the ink channel 31b and thus increases an ink pressure, resulting in an ejection of an ink droplet from an orifice (not shown). When the application of the electric field is stopped, restoration of the side walls occurs, so that the ink pressure in the ink channel is decreased and ink is supplemented into the ink channel from an ink supplier (not shown).
Manufacturing process of the ejection array will next be described with reference to FIG. 3. By way of grinding machining with a rotating diamond cutter disk, a plurality of grooves of rectangular cross-section are formed in parallel in the surface of the piezoelectric ceramic plate 1 which has been polarized in the direction of d. Next, the metal electrodes are formed on the surface of the grooves 3 by way of sputtering and then the cover plate 2 is bonded to the upper surfaces of the groove-formed ceramic plate 1. An orifice plate 4 formed with orifices 41 in positions corresponding to the ink channels is bonded to the side surface of the ceramic plate 1 at the ink ejection side.
In the conventional ejection device array as described above, there is a problem in that the neighboring ejection devices cannot be actuated simultaneously thus ink ejections from the neighboring ejection devices do not occur simultaneously. Because the side wall common to two neighboring ejection devices deforms in a push-pull fashion. Specifically, when the common side wall is inwardly deformed with respect to one of the two neighboring ink channel, the deformation direction of the same side wall is outward with respect to the other ink channel. Consequently, printing of characters or graphic images with such ejection device requires complicated control and the printing speed is slowed down.
Another problem with the conventional ink ejection device is that the device cannot be operated with a low driving voltage. This is because the piezoelectric ceramic plate forms the side wall and the bottom wall of the ink channel while serving as a piezoelectric actuator. More specifically, the efficiency of the piezoelectric thickness shearing effect is enhanced as a ratio of the side wall width of the ink channel to the height of the side wall, i.e., (width)/(height), becomes smaller. Further, the driving voltage can be lowered if the side wall width becomes thinner, because the electrode-to-electrode distance becomes smaller as the side wall width of the ink channel is thinner. As a matter of fact, however, it is extremely difficult to reduce the width of the side wall and to increase the height thereof. Weakness inheres in the material of the piezoelectric ceramic plate. Thus, the yieldability in the manufacture of the ejection devices is degraded and the strength of the manufactured product is lowered resulting in loss of reliability if the piezoelectric plate is manufactured so that the interval of the grooves is too small and/or the groove is too deep.